The methods of three-dimensional X-ray crystal structure determination will be applied to determine various aspects of metalloporphyrin stereochemistry. Metalloporphyrin to be studied include iron (II) and iron (III) complexes with a variety of axial ligands. These complexes are designed to model and extend our understanding of a number of hemoprotein systems, particularly to resolve certain unanswered questions about hemoprotein structure and function. Synthetic analogues of the cytochromes c and b will be studied to probe what structural features lead to their redox properties. Other complexes will have sterically hindered ligands and are to assess aspects of binding small ligands related to models of oxygen binding hemoproteins. Other studies are designed to study possible bridging ligands for multimetal systems such as cytochrome oxidase. Single crystal studies of nitrosyliron porphyrins are planned in order to investigate whether the ESR parameters are sensitive reporters of geometry at heme. Further studies on high-spin iron (II) porphyrins with coordination numbers other than five are planned. The investigation of the structural aspects of the interaction of O2 with nonredox metalloporphyrins is planned. Aspects of forced porphinato core conformations in metalloporphyrins and its relationship to other properties will be explored.